1. Field of the Invention
The present invention relates to a printing apparatus and method for a printer, a copy machine, a facsimile terminal equipment, or the like, and specifically, to correction of the deviation of a printed position resulting from an error in transportation of a printing sheet.
2. Description of the Related Art
Conventional printing apparatuses such as printers, copy machines, and facsimile terminal equipment are equipped with a mechanism which transports a printing sheet as a printing medium. The mechanism includes a transportation roller, a pinch roller pressing the printing sheet against the transportation roller and holding the printing sheet between the pinch roller and the transportation roller, a device for causing the pinch roller to apply a pressing force on the printing sheet, and other devices. Such transportation mechanism executes a transporting operation for the printing sheet fed by a sheet feeding section, in a printing area by a printing head, and two pairs of such transportation mechanisms are generally provided before and behind the printing area, respectively. Thus, the printing sheet is precisely transported in the printing area, and during the transportation, predetermined tension is applied to the printing sheet to keep it flat over a wide area.
FIG. 11 is a sectional view mainly showing the transporting mechanism for the printing sheet in a conventional example of a printing apparatus based on an ink jet method.
In the figure, a printing head 7 mounted in a carriage portion 5 executes a scanning operation in a direction perpendicular to the drawing sheet, and during the scanning operation, ejects ink for performing a printing operation. In relation to the printing area covered by the printing head, a printing sheet P is transported, under the carriage portion 5, from right to left in the figure while substantially keeping its horizontal position. More specifically, as the above-stated two pairs of transportation mechanisms, a pair of a transportation roller (hereinafter referred to as xe2x80x9cLF rollerxe2x80x9d) 36 and a pinch roller 37 is provided in an upstream side of the printing area, in which the printing sheet is transported, and a pair of a sheet discharging roller 41 and a spur 42 is provided in a downstream side of the printing area. Among these rollers, the pinch roller 37 is rotatably supported on a rotation shaft provided in a pinch roller holder 30. The pinch roller holder 30 is urged by a pinch roller spring 31 so that the pinch roller 37 can be pressed against the transportation roller 36. A pressing mechanism (not shown) similarly applies a pressing force which is applied between the sheet discharging roller 41 and the spur 42. The two pairs of rollers respectively hold the printing sheet P therebetween, and a driving mechanism (not shown) rotationally drives the transportation roller 36 and the sheet discharging roller 41, thereby causing the printing sheet P to be transported a predetermined distance for each one scanning operation of the printing head.
However, it is known that the above-described transportation mechanism may cause a deviation of transporting position of the printing sheet: when the printing sheet P is transported and a back end thereof slips out from the transportation roller 36 and the pinch roller 37 holding the printing sheet therebetween, the printing sheet P may be transported more than an expected predetermined distance, thereby a relative position of the printing head to the printing sheet P deviates from the regular one. As a result, a position (position of a printed image) of an ink dot formed on the printing sheet P with ink ejected from the printing head deviates from a standard position, thereby degrading the printed image.
FIGS. 12A and 12B show a positional relationship between the transportation roller 36 and the pinch roller 37. As shown in FIG. 12B, the transportation roller 36 has a length corresponding to a width of the printing sheet P. On the other hand, a plurality of pinch rollers 37, each of which is shorter than the transportation roller 36, are disposed correspondingly to the transportation roller. With this configuration, when the back end of the printing sheet P slips out from the transportation roller 36 and the pinch rollers 37, the pinch rollers 37 move toward the transportation roller a distance corresponding to a thickness of the printing sheet P, which has been held by the pinch rollers 37 and the transportation roller 37 between there. Urging force of the pinch roller 37 associated with this movement causes the printing sheet P to be transported an extra distance, that is, longer than the expected predetermined distance. At the same time, the transportation roller rotates an amount corresponding to the above extra transported distance.
To deal with such an error in transportation, it is considered that for example, a brake may be provided to stop rotation of the transportation roller to restrain the printing sheet P from being transported an extra distance when the sheet slips out. However, in this case, load torque required to drive the transportation roller increases, so that disadvantageously, a higher-grade drive motor must be used, and transportation speed cannot be increased.
The object of the present invention is to provide a printing apparatus and method that can use a simple configuration to reduce the deviation of a printed image position resulting from a behavior of the printing sheet when the back end of the printing sheet slips out from the upstream roller pair during transportation.
In a first aspect of the present invention, there is provided a printing apparatus which uses a printing head having a plurality of printing elements disposed thereon to perform printing on a printing medium, the apparatus comprising:
transportation means having a transportation roller and urging means for applying a pressing force on the printing medium to press the printing medium against the transportation roller, to perform transportation of the printing medium;
detection means for detecting an amount of rotation of the transportation roller to determine an amount of transportation of the printing medium; and
correction means for shifting an operative range of the printing element by using an inoperative printing element among the plurality of printing elements of the printing head correspondingly to the amount of transportation which the detection means detects when the pressing force applied on the printing medium by the urging means disappears, to correct a deviation of a position of the printing element to the printing medium, the deviation being caused by transporting of the amount of transportation detected by the detection means.
In a second aspect of the present invention, there is provided a printing method which uses a printing head having a plurality of printing elements disposed thereon to perform printing on a printing medium, the method comprising the steps of:
providing transportation means having a transportation roller and urging means for applying a pressing force on the printing medium to press the printing medium against the transportation roller, to perform transportation of the printing medium;
detecting an amount of rotation of the transportation roller to determine an amount of transportation of the printing medium; and
shifting an operative range of the printing element by using an inoperative printing element among the plurality of printing elements of the printing head correspondingly to the amount of transportation which the detecting step detects when the pressing force applied on the printing medium by the urging means disappears, to correct a deviation of a position of the printing element to the printing medium, the deviation being caused by transporting of the amount of transportation detected by the detecting step.
With the above configuration, during transportation of the printing medium, when the pressing force, by urging means for pressing the printing medium against the transportation roller in order to generate transportation force, applied on the printing medium disappears, that is, when the printing medium slips out from the transportation roller and the urging means, and even when the printing medium is transported an extra distance due to its slip-out, those of the plurality of printing elements of the printing head, which are usually not used for printing, are used to shift the operative range of the printing elements so as to correct the deviation of the positions of the printing elements relative to the printing medium correspondingly to a transportation detected by the detecting means. This prevents a position of a printed image from deviating from a predetermined position expected from an already printed image.